Conventionally, in digital communication, multimedia communication is performed through the following processes. That is, moving picture data, voice data and other data are compressed in conformity with a compression system of a prescribed standard; image data stream, voice data stream and other data stream that were obtained by the compression are multiplexed; and the multiplexed stream data is transmitted.
Before the transmission of the stream data, control data is exchanged between communication terminals in regard to a moving picture coding system, a voice coding system, a data transmission system, or the like. Then, after the communication system to be performed within the abilities of the communication terminals is determined, resources necessary for the communication are prepared. Not until these processes are completed the transmission of the stream data is begun.
As for the exchanges of the control data before the beginning of the transmission of the stream data, ITU-T standard H.245 (hereinafter, referred as to “H.245 standard”) is currently used. This standard is a communication control protocol that was developed for AV multimedia communication using packet multiplexing, and is adopted by many multimedia communication terminals.
In the H.245 standard, after the beginning of communication, at first, the exchanges of control data concerning the abilities of communication terminals and reception acknowledgment signals with regard to the control data are performed between communication terminals, and the exchanges of the information for the determination of the classification of the communication terminals and the superiority or inferiority of the communication terminals are performed. These exchange steps are called as “transmission starting phase 1”. In the transmission starting phase 1, exchanges of control data and reception acknowledgment signals concerning the control data are performed as many times as the number of media (voice, image, and so forth). After the transmission starting phase 1, the abilities of transmitter-receiver terminals are settled, and the logical connection of each medium is established. This step is called as “transmission starting phase 2”. After the completion of the transmission starting phase 2, the exchange of multiplexing information for the packet multiplexing of data and a reception acknowledgment signal with regard to the multiplexing information is performed. This exchange is called as “transmission starting phase 3”. Not until all of these processes of the transmission starting phases 1-3 are completed the transmission of the stream data is begun.
However, the H.245 standard adopts an acknowledgment type process configuration such that, after a communication terminal transmitted control data to a communicating party and received the reception acknowledgment signal (ACK, NACK) of the control data from the communicating party, the communication terminal transmits the next control data. Such a process configuration guarantees the sure transmission of control data. However, such a process configuration has a problem such that the larger the number of control data necessary for being exchanged before the beginning of the transmission of stream data becomes, the longer the necessary time before the beginning of the transmission of the stream data becomes. Incidentally, in the H.245 standard, generally, after about twenty control data were exchanged, the transmission of stream data is begun. Hereinafter, the problem is described in detail with the aid of FIG. 1.
FIG. 1 is a sequence diagram showing a state of exchanging of control data between communication terminals equipped with a conventional communication apparatus severally. In FIG. 1, at the timing (hereinafter, abbreviated to as “t”) 11 and t21, communication terminals A and B respectively transmit control data A-1 and control data B-1 to the other parties of communication. Incidentally, the problem is described in the following description on the basis of the operation of the communication terminal A for convenience of description.
After the communication terminal A transmitted the control data A-1, the communication terminal A generates control data A-2 to be transmitted at the next time. When the communication terminal B received the control data A-1 at t22, the communication terminal B transmits a reception acknowledgment signal 1 to the communication terminal A as a reply. When the communication terminal A received the reception acknowledgment signal 1 at t13, it becomes possible for the communication terminal A to transmit the next data. Accordingly, the communication terminal A transmits response data to an inquiry in the control data B-1 that the communication terminal A received at t12 to the communication terminal B at t13. Now, it is supposed that the preparation of the transmission of the control data A-2 to be transmitted from the communication terminal A at the next time has already completed at t13 in the communication terminal A.
When the communication terminal B received the response data at t23, the communication terminal B transmits a reception acknowledgment signal 2 to the communication terminal A as a reply. When the communication terminal A received the reception acknowledgment signal 2 at t14, it becomes possible for the communication terminal A to transmit the next data. Consequently, the communication terminal A transmits the control data A-2 the transmission preparation of which has completed at t13 at t14.
As described above, because the communication terminal A cannot transmits control data A-2 before the reception of the reception acknowledgment signal 2 in the 1H.245 standard, awaiting time shown in FIG. 1 is generated with respect to the transmission of the control data A-2.
The larger the number of control data necessary to be exchanged before the beginning of the transmission of stream data becomes, the higher the frequency of generation of such a waiting time becomes. Consequently, the larger the number of control data becomes, the longer the necessary time before the beginning of the transmission of stream data becomes.